Hi! I found this spell http://www.spellsofmagic.com/spells/wealth_spells/fortune_spells/250/page.html here around on website. You have to chant the following text and learn well you will get "nothing less than A".

"On this test I take today, I will receive no less than A. Earth, Wind, Fire and Sea, As I say, So mote it Be!"

Well in my country there aren't grades such as "ABCD..." only 1 to 10. If I will change A in spell into 10, it will help me get this 10?

the spell places quite a strain on the universe, so ebar that in mind when altering it. for example dont expect teh spell to jump you up five grade boundaries if you dont put any actual study in too. and try to aim for a grade thats just out of reach. its a pretty simple spell, and you may need a psychodrama that is more involved to get miracles.
but in response to the question, if you re-word the spell to suit you that will be fine, as long as you percieve the spell as having power. try to make it rhyme, the second part does so the first part may sound silly if it doesnt, and you dont want to feel silly when youre casting.
my other advice is to just repeat the chant over and over until something 'clicks', doing this will make you feel like you are doing something important, and that will get your magic flowing. repetition also 'paralyses the psychic censor' preventing your own internal critique from disrupting the spell.
sorry for the long reply, i wanted it to be helpful :P